1. Field of the Invention
This invention relates to an apparatus for inspecting objects such as negatives, and in particular to an apparatus for detecting defects such as non-transmitting dust other than the pattern for exposure of a reticle or a photomask used in the semiconductor manufacturing process.
2. Description of the Prior Art
Generally in the IC manufacturing process, a desired pattern for exposure of a reticle or a mask is transferred to a semiconductor wafer, having resist applied thereto, by the use of the projection optical system in a semiconductor printing apparatus (stepper or other mask aligner) or by the use of intimate contact process.
When the pattern is to be transferred from the reticle or mask to the wafer having resist applied thereto by the semiconductor printing apparatus, if a defect such as dust is present on the reticle or mask, the shape of the defect in addition to the original pattern of the reticle or mask will be printed and this may cause a reduced yield of the IC manufacture.
Particularly, where use is made of a "stepper" which uses a reticle to print a desired pattern on a wafer by step and repeat, a piece of dust on the reticle would be printed on the entire surface of the wafer.
Thus, in recent years, the presence of dust has become a great problem. Various methods of inspecting a defect of the pattern of a reticle or mask have heretofore been used.
These methods include "the method of comparing with the design data" and "the method of comparing adjacent chips", and are used for an automatic defect inspecting apparatus. "The method of comparing with the design data" is a method which comprises prestoring the design data which is the ideal pattern of a reticle or mask so that the design data can be processed by a computer, irradiating the reticle or mask with a laser or the like, and comparing the pattern from the transmitted light thereof with the design data to thereby detect any defect, and this method is also capable of detecting a common defect of the chips on the mask resulting from malfunctioning of a pattern generator or the like.
In this method, however, the design data handled is enormous and therefore, a long inspection time is required and high accuracy is required of the positioning of the object to be inspected.
"The method of comparing adjacent chips" comprises comparing the chip patterns on a mask to thereby detect any defect and therefore, it eliminates the necessity of using any other object of comparison than the object to be inspected, such as the design data, and only requires a short inspection time.
However, this method cannot inspect in principle in the case of one chip per reticle for a stepper.
Accordingly, neither of these prior art methods is suitable as the method for inspecting the reticle of a stepper. This is particularly so when an automated dust inspecting apparatus which requires high-speed inspection is considered.